The present invention relates to a servo system in a data storage device and, in particular, to demodulation of position error signals (PES) within the servo system.
A data storage device, such as a magnetic disc drive, stores data on a recording medium The recording medium is typically divided into a plurality of generally parallel data tracks. In a magnetic disc drive, the data tracks are arranged concentrically with one another, perpendicular to the disc radius. The data is stored and retrieved by a transducer or "head" that is positioned over a desired data track by an actuator arm.
The actuator arm moves the head in a radial direction across the data tracks under the control of a closed-loop servo system based on servo data stored on the disc surface within dedicated servo fields. The servo fields can be interleaved with data sectors on the disc surface or on a separate disc surface that is dedicated to storing servo information. As the head passes over the servo fields, it generates a readback servo signal that identifies the location of the head relative to the centerline of the desired track. Based on this location, the servo system rotates the actuator arm to adjust the head's position so that it moves toward a desired position.
There are several types of servo field patterns, such as a "null-type" servo pattern, a "split-burst amplitude" servo pattern, and a "phase type" servo pattern. A null type servo pattern includes at least two fields which are written at a known phase relation to one another. The first field is a "phase" or "sync" field which is used to lock the phase and frequency of the read channel to the phase and frequency of the read signal. The second field is a position error field which is used to identify the location of the head with respect to the track centerline.
As the head passes over the position error field, the amplitude and phase of the read signal indicates the magnitude and direction of the head offset with respect to the track centerline. The position error field has a null-type magnetization pattern such that when the head is directly straddling the track centerline, the amplitude of the readback signal is ideally zero. As the head moves away from the desired track centerline, the amplitude of the read signal increases. When the head is half-way between the desired track centerline and the centerline of the adjacent track, the read signal has a maximum amplitude. The magnetization pattern on one side of the centerline is written 180.degree. out of phase with the magnetization pattern on the other side of the centerline. Thus, the phase of the read signal indicates the direction of the head position error.
To control the servo system, a single position error value is generated for each pass over the position error field. Typically, the magnitude of the position error value indicates the distance of the head from the track centerline, and the sign of the position error value indicates the direction of the head's displacement. The position error values are typically created by demodulating the read signal associated with the position error field. In a synchronous process, the exact phase of the read signal from the position error field is known from the phase field's read signal because the phase field is written on the storage medium at a known and fixed phase relation to the position error field. A phase-locked loop (PLL) is typically used to acquire the phase of the phase field, and this phase information is used for demodulating the position error field.
The read signal is demodulated by generating a demodulating signal, such as a square wave, having the same phase and frequency as a fundamental component of the read signal and then, with analog techniques, multiplying the read signal by the demodulating signal. The product is integrated over a time window that corresponds to the middle cycles of the position error field. The result is a position error value for the head with respect to a desired position on the storage medium within that servo pattern. This process essentially identifies the amplitude and phase of the read signal at a specific frequency point. The sign of the position error value indicates which direction the head is located with respect to the desired location.
Although demodulating the read signal with analog techniques provides a very accurate position error value, this type of demodulation can be difficult to integrate onto the same integrated circuit as a predominately digital data channel. This can increase the overall cost and complexity of a servo system.
The present invention addresses these and other problems, and offers other advantages over the prior art.